This invention relates to the calibration of an oscillator which forms part of a phase-locked loop (PLL).
U.S. Pat. No. 5,408,196 describes a receiver which includes a PLL for demodulating an FM signal. In the receiver, a band-pass filter is coupled to a signal input of a phase detector which forms a part of the PLL. In a calibrating state, a switch connects a signal source to the band-pass filter. The signal source preferably provides a broadband noise signal. There is an overlap between the passband of the band-pass filter and the spectrum of the signal source. Therefore, the signal source will generate, via the band-pass filter, a signal with a spectrum in the passband of the band-pass filter at the signal input of the phase detector. In response to this signal, the PLL will draw the frequency of its oscillator towards the passband. At the end of the calibration a lock signal which is developed at the control input of the PLL""s oscillator, is stored in a memory. This lock signal will be supplied as a tuning signal to the PLL""s oscillator, and will tune it within the passband.
The invention seeks, inter alia, to provide for the calibration of an oscillator forming part of a PLL which, with respect to the background art, is better suited for a wide variety of applications. Claims 1, 6 and 7 define, respectively, a receiver, an integrated circuit and a method of calibrating an oscillator which forms part of a PLL, in accordance with the invention. Additional features, which may be optionally used to implement the invention to advantage, are defined in the dependent claims.
The invention takes the following aspects into consideration. A PLL may receive a carrier via a filter whose centre frequency may not correspond to the nominal frequency of the carrier. For example, let it be assumed that a PLL is used for demodulating a video-modulated carrier in a television (TV) receiver suited for conventional terrestrial TV broadcast signals. In that case, the PLL will receive the video-modulated carrier via a filter which has a Nyquist slope at one end of its passband. The nominal frequency of the video-modulated carrier will substantially be at the centre of the Nyquist slope. However, the centre frequency of the filter will be substantially half-way the two ends of the passband which is, for example, 6 MHz wide. That is, the centre frequency of the filter will be located approximately 3 Mhz away from the nominal frequency of the video-modulated carrier.
In the background art, the frequency of the PLL""s oscillator will substantially correspond to the centre frequency of the band-pass filter when the calibration has been completed. If the nominal frequency of a carrier does not substantially correspond to the centre frequency, it may prevent the PLL from capturing the carrier. For example, in a TV receiver application as described above, the PLL""s oscillator will be approximately 3 MHz away from the nominal frequency of the video-modulated carrier if it is calibrated in accordance with the background art. The PLL will preferably have a relatively small loop bandwidth, for example in the order of a few kHz, to prevent parasitic modulation of the PLL""s oscillator which may cause interference. With such a small loop bandwidth, the PLL will not be able to capture the video-modulated carrier which is at a distance in frequency of 3 MHz. Rather, it will parasitically capture one of the sidebands of the video-modulated carrier, in particular, a sideband which is a multiple of the line frequency used.
In accordance with the invention, a frequency difference between the nominal frequency of the carrier and the frequency of the PLL""s oscillator is measured, and the latter is adjusted in accordance with the measured frequency difference. As a result, the PLL""s oscillator will be relatively close in frequency to the carrier which is supplied to the PLL. Consequently, the PLL will be able to capture the carrier without great difficulty, even if the PLL has a relatively small loop bandwidth. Moreover, in this respect, it does not matter via which type of filter, if any, the carrier is supplied to the PLL. Thus, the invention is better suited for a wide variety of applications than the background art.
Another advantage of the invention with respect to the background art relates to the following aspect. In the background art, the signal source and the switch, which form part of the circuitry for calibrating the PLL""s oscillator, are separated from the PLL by the filter via which the carrier is supplied to the PLL. In many cases, it will be difficult, if not impossible, to realize the filter in the form of an integrated circuit. If, nevertheless, the signal source and the switch are realized in an integrated circuit together with the PLL, certain problems may arise. For example, there may be insufficient filtering due to a parasitic capacitance between two pins of the integrated circuit, one of the two pins being coupled to an input of the filter and the other pin to an output.
In contradistinction, in the invention, the PLL as well as the circuitry for calibrating the PLL""s oscillator may form a part of a single integrated circuit without this leading to any appreciable problems. Thus, the invention is better suited for implementation on a single integrated circuit than the background art. Such an implementation generally contributes to cost-efficiency.